In recent years, in response to strong social demands for low fuel consumption of vehicles, weight reduction of tires has been sought. Among tire members, weight reduction and the like of an inner liner also have begun. The inner liner is disposed inwardly in a tire, and has a function of reducing an amount of leakage of air from inside to outside of the pneumatic tire.
Currently, a rubber composition for such an inner liner employs, for example, a rubber blend mainly containing a butyl rubber. The rubber blend contains the butyl rubber by 70 mass % to 100 mass %, and a natural rubber by 30 mass % to 0 mass %. In this way, the tire is provided with improved air permeability resistance. In addition to butylene, the rubber blend mainly containing the butyl rubber contains isoprene by approximately 1 mass %, which acts with sulfur, vulcanization accelerator, and zinc white to achieve cross-linking between rubber molecules. In the case of normal blend, the above-described butyl-based rubber needs to have a thickness of 0.6 mm to 1.0 mm for a tire of a passenger car, and needs to have a thickness of approximately 1.0 mm to 2.0 mm for a tire of a truck/bus. In order to achieve weight reduction of such tires, use of polymer, which is more excellent in air permeability resistance than the butyl-based rubber and can provide an inner liner layer with a thinner thickness, has been requested.
Conventionally, in order to achieve weight reduction of a tire, it has been proposed to use a film made of a material including thermoplastic resin, instead of the above-described rubber composition. However, when a tire is manufactured using an inner liner of thin thermoplastic resin, the inner liner partially becomes too thin due to pressure in a vulcanization step, with the result that the finishing gauge of the inner liner in the resulting tire product becomes thinner than the designed gauge. In the thin portion of the inner liner thus finished, a phenomenon (open thread) in which a carcass cord looks to stand out takes place, thus giving a user an impression of bad inner appearance. In addition, when the inner liner is thin, gas barrier property becomes partially bad to decrease tire internal pressure, with the result that the tire may burst in the worst case.
Meanwhile, during traveling with the tire, large shear strain acts on a vicinity of a shoulder portion in the inner liner. When the material including the thermoplastic resin is used as the inner liner, this shear strain is likely to cause detachment at an adhesion interface between the inner liner and the carcass ply, with the result that air leakage takes place from the tire, disadvantageously.
In Patent Literature 1 (International Publication WO2008/029781), a tire is manufactured using a strip of film layer stack in which thermoplastic resin and thermoplastic elastomer are blended. With the film layer stack, gas barrier property and adhesive property can be improved, whereby bonding can be achieved between the ribbon-shaped strips. However, in this technique, gauge is constant in a non-vulcanized raw cover of film layer stack. Hence, when the gauge is thinned, a buttress portion or the like in the finished tire after vulcanization may become thin.
Patent Literature 2 (Japanese Patent Laying-Open No. 2009-220460) discloses a method for manufacturing an inner liner material by extrusion molding a thermoplastic elastomer composition in which a thermoplastic resin constitutes a sea component and a rubber constitutes an island component into a sheet shape through an extrusion mouthpiece, wherein the extrusion mouthpiece has a mouthpiece slit having a thick extrusion portion in cross section between the central part of the slit and each of opposite ends of the slit and has a ratio (%) of thickness increment Δt with respect to length Δ1 of a thickness changing portion in the slit longitudinal direction is set at 0.01 to 10%. With this structure, it is intended to obtain characteristics of enhanced effect of preventing air leakage and of unlikeliness to peel off.
However, the size of the extrusion die is difficult to change, which imposes limits on the size of tires to be manufactured. Even if several types of extrusion dies are prepared, it will take time for stage changing at the time of size changing, which decreases productivity.
Patent Literature 3 (Japanese Patent Laying-Open No. 2000-254980) discloses sequentially winding a ribbon-shaped unvulcanized rubber strip on a cylindrical drum, thereby forming a rubber component into a contour close to a desired finished sectional shape.
Conventionally, an inner liner used for a pneumatic tire is obtained in general by continuously extrusion molding into a predetermined finished sectional shape from a rubber extruder or the like. The finished sectional shape is determined by a mouthpiece provided at a head part of the rubber extruder. In the conventional method for extrusion molding into a finished sectional shape, the sectional size of a rubber component is large, so that a large size rubber extruder needs to be used. As a result, a production line cannot be reduced in size. Moreover, to solve problems of deteriorated productivity in small quantity, large variety production and the like, various types of mouthpieces should be prepared depending on the types of tire and the like, and besides, exchanging and adjusting operations of the mouthpiece and the like are required every time the type of tire to be manufactured is changed.
However, when forming a tire component by a ribbon-shaped rubber strip, workability is disadvantageous due to tackiness between rubber compositions and the rubber component formed of the rubber strip deforms disadvantageously during storage.
Patent Literature 4 (Japanese Patent Laying-Open No. 2010-058437) discloses a method for molding a sheet by extruding melted resin into a sheet shape from a die, holding the extruded resin sheet between a die roller and a nip roller, at least one of which has a projecting shape formed thereon, to transfer the projecting shape to the sheet, forming a cut groove therein, and cooling and hardening.
Patent Literature 5 (Japanese Patent Laying-Open No. 9-19987) discloses a layer stack for improving adhesive property between an inner liner layer and a rubber layer. By providing adhesion layers on the opposite sides of the inner liner layer, the adhesion layers come into contact with each other at an overlapping portion of the inner liner layer and are bonded firmly by heating. Air pressure retainability is thus improved. However, these adhesion layers for overlapping in the inner liner layer will come into contact with a bladder in a heated state in a vulcanization step and will be stuck to the bladder disadvantageously.